MQ(3) BSD Library Functions Manual MQ(3)NAME
mq, mqueue -- POSIX message queues (REALTIME)
LIBRARY
POSIX Real-time Library (librt, -lrt)
SYNOPSIS
#include <mqueue.h>
DESCRIPTION
The IEEE Std 1003.1-2001 (``POSIX.1'') standard defines and NetBSD implements an interpro-
cess communication (IPC) interface known as POSIX message queues. Although the basic func-
tionality is similar, mq is distinct from the older AT&T System V UNIX message queues (see
for example ipcs(1) or msgget(2)).
Rationale
The rationale behind mq is to provide an efficient, priority-driven asynchronous IPC mecha-
nism. When the AT&T System V UNIX message queues were first implemented, the reasoning was
similar: the only form of IPC was half-duplex pipes and message queues were seen to overcome
the performance limitations with these.
But arguably in modern systems there is little difference between the efficiency of the Sys-
tem V message queues, pipes, and UNIX domain sockets (if anything, the AT&T System V UNIX
message queues tend to be slower than the rest). The fundamental performance bottleneck is
however still there with mq as well: data must be first copied from the sender to the kernel
and then from the kernel to the receiver. The bigger the message, the higher the overhead.
For realtime applications, mq offers some advantages:
1. Unlike the predecessors, mq provides an asynchronous notification mechanism.
2. Messages are prioritized. The queue always remains sorted such that the oldest mes-
sage of the highest priority is always received first, regardless of the number of
messages in the queue.
3. By default, the functions to send and receive messages are blocking calls. It is
however possible to use non-blocking variants with mq. Furthermore, it is possible
to specify timeouts to avoid non-deterministic blocking.
4. Resource limits can be enforced -- or perhaps more importantly, the availability of
resources can be ensured as the internal data structures are preallocated.
Descriptors and Naming
Comparable to pipes and FIFOs (a.k.a. named pipes), all POSIX message queue operations are
performed by using a descriptor. The used type is mqd_t, an abbreviation from a ``message
queue descriptor''. In the NetBSD implementation this is actually an ordinary file descrip-
tor. This means that it is possible, but not portable, to monitor a message queue descrip-
tor by using poll(2) or select(2).
Message queues are named by character strings that represent (absolute) pathnames. The used
interface is analogous to the conventional file concepts. But unlike FIFOs and pipes, nei-
ther POSIX nor System V message queues are accessed by using open(2), read(2), or write(2).
Instead, equivalents such as mq_open(), mq_close(), and mq_unlink() are used.
The standard does not specify whether POSIX message queues are exposed at the file system
level. It can be argued that mq inherited an old problem with the System V message queues.
Even if an implementation would have support for it, it is not portable to view message
queues by ls(1), remove these with rm(1), or adjust the permissions with chmod(1).
Processes
When a new process is created or the program is terminated, message queues behave like
files. More specifically, when fork(2) is called, files and message queues are inherited,
and when the program terminates by calling exit(3) or _exit(2), both file descriptors and
message queues are closed. However, the exec(3) family of functions behave somewhat differ-
ently for message queues and files: all message queues are closed when a process calls one
of the exec() functions. In this respect POSIX message queues are closer to FIFOs than nor-
mal pipes.
Attributes
All message queues have an attribute associated with them. This is represented by the
mq_attr structure:
struct mq_attr {
long mq_flags;
long mq_maxmsg;
long mq_msgsize;
long mq_curmsgs;
};
The members in the structure are: flags set for the message queue (mq_flags), the maximum
number of messages in the queue (mq_maxmsg), the maximum size of each message (mq_msgsize),
and the number of queued messages (mq_curmsgs).
The overall resource requirements for a particular message queue are given by mq_maxmsg and
mq_msgsize. These two can be specified when the queue is created by a call to mq_open().
The constraints are enforced through the lifetime of the queue: an error is returned if a
message larger than mq_msgsize is sent, and if the message queue is already full, as deter-
mined by mq_maxmsg, the call to queue a message will either block or error out.
Although there are two functions, mq_getattr() and mq_setattr(), to retrieve and set
attributes, resource limits cannot be changed once the queue has been created. In NetBSD
the super user may however control the global resource limits by using few sysctl(7) vari-
ables.
Asynchronous Notification
Instead of blocking in the functions that receive messages, mq offers an asynchronous mecha-
nism for a process to receive notifications that messages are available in the message
queue. The function mq_notify() is used to register for notification. Either a signal or a
thread can be used as the type of notification; see sigevent(3) for details.
Bear in mind that no notification is sent for an arrival of a message to a non-empty message
queue. In other words, mq_notify() does not by itself ensure that a process will be noti-
fied every time a message arrives. Thus, after having called mq_notify(), an application
may need to repeatedly call mq_receive() until the queue is empty. This requires that the
message queue was created with the O_NONBLOCK flag; otherwise mq_receive() blocks until a
message is again queued or the call is interrupted by a signal. This may be a limitation
for some realtime applications.
Priorities
Each message has a priority, ranging from 0 to the implementation-defined MQ_PRIO_MAX. The
POSIX standard enforces the minimum value of the maximum priority to be 32. All messages
are inserted into a message queue according to the specified priority. High priority mes-
sages are sent before low priority messages. If the used priority is constant, mq follows
the FIFO (First In, First Out) principle.
The basic rule of thumb with realtime prioritization is that low priority tasks should never
unnecessarily delay high priority tasks. Priority inheritance is not however part of the
provided API; the receiver process may run at low priority even when receiving high priority
messages. To address this limitation and other potential realtime problems, the user may
consider other functions from the POSIX Real-time Library (librt, -lrt). The process sched-
uling interface described in sched(3) can be mentioned as an example.
FUNCTIONS
The following functions are available in the API.
Function Description
mq_open(3) open a message queue
mq_close(3) close a message queue
mq_unlink(3) remove a message queue
mq_send(3) send a message
mq_receive(3) receive a message
mq_timedsend(3) send a message with a timeout
mq_timedreceive(3) receive a message with a timeout
mq_getattr(3) get message queue attributes
mq_setattr(3) set message queue attributes
mq_notify(3) register asynchronous notify
COMPATIBILITY
Despite of some early fears, the POSIX message queue implementations are fairly compatible
with each other. Nevertheless, few points can be noted for portable applications.
o It is not portable to use functions external to the API with message queue descriptors.
o The standard leaves the rules loose with respect to the message queue names. Only the
interpretation of the first slash character is consistent; the following slash charac-
ters may or may not follow the conventional construction rules for a pathname.
o The length limits for a message queue name are implementation-defined. These may or may
not follow the conventional pathname limits PATH_MAX and NAME_MAX.
SEE ALSO
Bill O. Gallmeister, POSIX.4: Programming for the Real World, O'Reilly and Associates, 1995.
Richard W. Stevens, UNIX Network Programming, Volume 2: Interprocess Communications,
Prentice Hall, Second Edition, 1998.
STANDARDS
The POSIX message queue implementation is expected to conform to IEEE Std 1003.1-2001
(``POSIX.1'').
HISTORY
The POSIX message queue API first appeared in NetBSD 5.0.
CAVEATS
User should be careful to unlink message queues at the program termination. Otherwise it is
possible to leave them lying around.
BSD July 28, 2010 BSD